Fill level measurement devices of this type are known per se; they are usually mounted above the medium, also called “fill material”, in, or at, the container. For determining the fill level of the medium, high frequency signals, ultrasonic signals or optical signals generated in the fill level measurement device are sent from the measurement device in the direction onto the medium, where they are reflected at a surface of the medium. These reflected signals, also called the useful signals, are registered by the fill level measurement device. Their travel time is a measure for the distance of the measurement device from the reflecting surface of the medium. Combined with knowledge of the container geometry, the fill level of the medium in the container can be determined.
Known fill level measurement devices of the described kind include, for example, fill level measurement devices which work with microwave- or radar-measurement signals, wherein the radar signals are radiated from an antenna freely onto the medium and can be received as they come back from there. Other fill level measurement devices with radar measurement signals are, for example, those in which the radar measurement signals are guided by a waveguide extending into the fill material.
A major problem for the mentioned fill level measurement devices, which work on the principle of travel time, is that the fill level measurement devices not only receive reflected signals from the fill material surface but also undesired reflections from so-called interference locations in the container. These interference signals, which, for example, are caused by objects installed in the container or by the container geometry, can be superimposed on the actually desired useful signals from the fill material surface in such a way that the useful signals can no longer, or not unequivocally, be identified during an evaluation of the measurement signals.
On the other hand, the interference signals provide a wealth of information, by which the functioning of the measurement device can be reviewed and with which additional information concerning the fill material (information such as dielectric constant, conductivity, moisture content, temperature, mixing ratio, foam formation, phase separation) can be obtained. However, this has not yet happened in this form.
In DE-A-42 33 324, for example, a freely radiating radar device with transmitting and receiving antenna is described. In this case, a signal from the floor of the container is used, in order to determine the fill level of the medium in the container, in the face of a fill level signal which cannot be identified unequivocally. The method described there solely for measurement of liquids assumes, however, a quite accurate knowledge of the dielectric constant and, in some cases, the magnetic permeability of the fill material, in order to determine the unknown fill level. This kind of information is not always present, however, and the physical properties of the medium in the container can also change. In view of this situation, this reference proposes to use the time displacement of the floor signal either to calculate the fill level directly, or else, in the evaluation of the signals of the measurement curve, to place a window over the remaining signals and to identify the fill level signal in this window.
In EP-A-0 457 801, an interference location in the stilling tube is proposed for calibration of a fill level measurement device with guided radar signals, wherein expressly polarizable signals are required and the polarization plane is changeable by a polarization device, whereby the intensity of the interference reflection at the interference location can be changed. Use of a polarization device is not only costly and an extra expense; often the signals cannot even be polarized, for example in the case of single-wire- or coaxial-waveguides of a TDR fill level measurement device.
In WO 00/43806, a TDR fill level measurement device is described, in which the dielectric constant can be determined in the context of an interface measurement. Discussion is of two reflecting surfaces as two “product boundary layers” of the fill material, where one of the interference signals produced thereby can even be generated by the end of the waveguide or by the floor of the container. This reference does discuss a calculation of the dielectric constant of the medium, but the calculation contains error.
Additionally, fill level measurement devices of the past, with guided radar measurement signals, have already included waveguides, whose geometry exhibits given reflection locations, thus known interference locations. However, a comprehensive evaluation of the signals delivered therefrom in the above-mentioned direction has, to this point, not happened.
Thus, for example, U.S. Pat. No. 3,474,337 describes a fill level measurement device with a wave-guide conducting radar signals. This is a so-called “TDR-system (Time Domain Reflectometry)”, in which geometric reflection locations in a coaxial waveguide are used as reference locations. The subject matter described and protected in this US patent includes, however, without exception, waveguides having at least two separated guides and otherwise is described in relatively general terms. This patent discloses no possibility of determining, from only the reflections of the interference locations, the fill level or some other characteristic. Interference signals are evaluated only in connection with reflections from the fill material.